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doi:10.2204/iodp.proc.333.102.2012

X-ray computed tomography

X-ray computed tomography (CT) imaging provided information about structures and sedimentological features in the core and helped to assess sample locations and quality for whole-round samples. Our methods followed those in the measurement manual prepared by the Center for Deep Earth Exploration (CDEX) (X-ray CT Scanning, version 1.00, 26 Dec 2008) and used on previous expeditions (e.g., IODP Expeditions 315, 316, 319, 322, and 331). The manual is based on GE Healthcare (2006), Mees et al. (2003), and Nakano et al. (2000).

A LightSpeed Ultra 16 (GE Yokogawa Medical Systems, Ltd.), capable of generating sixteen 0.625 mm thick slice images every 0.5 s, the time for one revolution of the X-ray source around the sample (Table T1), is the X-ray CT scanner on the Chikyu. Data generated for each core consist of core-axis-normal planes of X-ray attenuation values with dimensions of 512 × 512 pixels. Data were stored as Digital Imaging and Communication in Medicine (DICOM) formatted files.

Background

The theory behind X-ray CT has been well established through medical research and is very briefly outlined here. X-ray intensity varies as a function of X-ray path length and the linear attenuation coefficient (LAC) of the target material as

where

• I = transmitted X-ray intensity,

• I₀ = initial X-ray intensity,

• µ = LAC of the target material, and

• L = X-ray path length through the material.

LAC is a function of the chemical composition and density of the target material. The basic measure of attenuation, or radiodensity, is the CT number given in Hounsfield units (HU) and is defined as

where

• µ_t = LAC for the target material, and

• µ_w = LAC for water.

The distribution of attenuation values mapped to an individual slice comprises the raw data that are used for subsequent image processing. Successive 2-D slices yield a representation of attenuation values in 3-D voxels.

Analytical standards used during Expedition 333 were air (CT number = –1000), water (CT number = 0), and aluminum (2477 < CT number < 2487) in an acrylic core mock-up. All three standards were run once daily after air calibration. For each standard analysis, the CT number was determined for a 24.85 mm² area at fixed coordinates near the center of the cylinder.

X-ray CT scan data usage

X-ray CT scans were used during Expedition 333 to

• Examine 3-D features of deformation structures, bioturbation, and so on;

• Distinguish “natural” fracture/faults and drilling-induced fractures;

• Measure dip angles of structures such as faults, bedding, veins, and so on;

• Provide an assessment of core and core liner integrity;

• Determine locations for whole-round samples; and

• Identify important structural and sedimentological features to be avoided by whole-round sampling.

X-ray CT scanning was done immediately after core cutting for time-sensitive (interstitial water, microbiology, and organic geochemistry) samples to finalize their selection. All whole-round core sections were screened to avoid destructive testing on intervals that may contain interesting structural or sedimentological features. This also facilitated identifying intervals with minimal drilling disturbance for whole-round sampling and for assessing heterogeneity (essential for postexpedition studies of frictional, geotechnical, and hydrogeological properties).

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